This invention relates to a piston ring and, particularly, to a piston ring having improved sealing characteristics with respect to a gap thereof.
Conventionally, a piston ring fitted in an annular groove in a piston and working in a cylinder has a gap between the circumferentially opposite ends of the piston ring to permit the thermal expansion of the piston ring and to permit the wear of the cylinder and the piston ring. When a pressure difference acts between the axially opposite surfaces of the piston ring, gas leaks therebetween through the radial clearance between the inner wall of the cylinder and the outer circumferential surface of the piston ring and through the gap between the circumferential opposite ends of the piston ring. When the dimension of the gap increases the gas leakage through the gap increases as compared with the leakage through the remaining portion.
Various proposals have been made to decrease the gas leakage through the gap, e.g., the circumferentially opposite ends of the piston ring are formed to have stepped configurations which partially overlap with each other in the axial direction whereby the effective area of the gap causing the gas leakage can be reduced by about one half.
Another proposal includes a metal backing ring which is inserted between the inner circumferential surface of the piston ring and the bottom wall of the piston ring groove and is pressed against the inner circumferential surface of the piston ring, particularly at the location of the gap, by the resiliency of the backing ring. The backing ring is effective in reducing substantially the gas leakage through the gap. However, provision of the backing ring is not necessarily advantageous since the backing ring tends to excessively press the piston ring against the cylinder wall thereby increasing the wear of the piston ring and the cylinder, and the piston ring groove will sometimes be damaged by the backing ring. Further, a backing ring is a member separate from the piston ring, thus complicating the manufacturing and assembling process.
Japanese Patent Publication No. 54-42083 shows a resilient non-metallic seal ring or arcuate plate interposed between the inner surface of the piston ring and the bottom wall of the piston ring groove so as to reduce the pressing force applied on the piston ring. The damage to the piston ring groove can be prevented, but desired sealing characteristics cannot necessarily be obtained since the seal plate will sometimes separate from the piston ring.
Japanese Patent Disclosure (Kokai Koho) No. 55-27524 shows a piston ring having axially overlapping stepped ends. The stepped ends of the low pressure side of the piston ring are further formed to have radially overlapping stepped configurations, whereby the gap in the low pressure side is divided into two circumferential gaps and a circumferentially extending clearance, thus making it possible to reduce the leakage through the gap. However, the circumferentially extending clearance will increase due to the difference in the wear in the adjacent end portions of the piston ring and thus the sealing characteristics of the sealing ring will be impaired during use.